The present invention relates to methods of forming multi-level ceramic packages for micro-electronic applications, more particularly an improved method for eliminating problems caused by dimensional instability of green ceramic sheets prior to sintering.
A common method for fabricating multi-level ceramic micro-electronic structures consists of forming ceramic material into a flexible tape, cutting the tape into sheets, referred to as green sheets, forming via holes at predetermined locations in the separate sheets, depositing conductive paste on the desired areas of the separate sheets and in the via holes, stacking the sheets upon one another, registering them, and subjecting them to relatively high sintering temperatures for a period of time long enough to burn out the organic binding material and to subsequently reforming the ceramic particles into the solid unit which is substantially impervious. During sintering the ceramic sheets undergo relatively large shrinkage which must be accomodated at earlier stages in the process to obtain the desired finished size. Subsequently top and bottom metallurgy patterns are applied to these unitary sintered package substrate and pins or other fixtures attached for making external connections. Semiconductor devices are also attached to the top side by a suitable technique, preferably by flip-chip bonding. An automated embodiment of this process is described and claimed in U.S. Pat. No. 3,518,765.
The ceramic green sheet, before it is ultimately laminated and sintered, must undergo a significant amount of processing. This processing, which includes the forming of via holes, filling of the via holes with conductive paste, and the forming of circuit patterns, takes significant amounts of time. Add to this the additional time it takes to cut the green ceramic sheet as it comes from the doctor blade castor, inspecting the sheets, punching alignment holes and then the related sorting and cataloging operations and the time that the green ceramic sheet remains in this form becomes quite lengthy. Green ceramic sheet is inherently dimensionally unstable since it includes volatile organic resin solvents and plasticizers which can evaporate and change the overall composition and also with it the dimensions. Further it can also absorb moisture, and/or possibly react with atmospheric elements. Thus the dimensional instability can present very serious problems, particularly when the technology embodies very small conductive metallurgy and closely spaced holes. Dimensional changes may result in misalignment of metal patterns on a green sheet and the conductive material in the via holes of the overlying ceramic green sheet.